Markus Karahka

Do Dielectric Nanostructures Turn Metallic in High-Electric dc Fields?

Three-dimensional dielectric nanostructures have been analyzed using field ion microscopy (FIM) to study the electric dc field penetration inside these structures. The field is proved to be screened within a few nanometers as theoretically calculated taking into account the high-field impact ionization process. Moreover, the strong dc field of the order of 0.1 V/Å at the surface inside a dielectric nanostructure modifies its band structure leading to a strong band gap shrinkage and thus to a strong metal-like optical absorption near the surface. This metal-like behavior was theoretically predicted using first-principle calculations and experimentally proved using laser-assisted atom probe tomography (APT). This work opens up interesting perspectives for the study of the performance of all field-effect nanodevices, such as nanotransistor or super capacitor, and for the understanding of the physical mechanisms of field evaporation of dielectric nanotips in APT.

Field evaporation of oxides: a theoretical study.

To understand atom probe results on the field evaporation of oxides we use density functional theory on MgO clusters to follow the structural changes during field evaporation and toobtain potential energy curves, partial charges and desorption pathways. It is straightforward to understand that Mg evaporates doubly charged. We also show that MgO(+), MgO₂(+), MgO(2+) and O(+) ions leave the surface. Two questions are however new for oxides. (1) Where do the electrons go? When the oxides are deposited on a metal tip it can be assumed that the electrons are used to complete the electrical circuit. However this leaves the second question unanswered, namely (2) what happens to the oxygen? We will argue that there are two channels for the oxygen, namely (a) To travel down the (metallic) surface of the tip and eventually to desorb either as atoms or molecules. (b) The oxygen can recombine within the oxide layer itself and desorbs as a neutral molecule accelerated in the inhomogeneous field due to its induced dipole.

Field evaporation of ZnO: A first-principles study

With recent advances in atom probe tomography of insulators and semiconductors, there is a need to understand high electrostatic field effects in these materials as well as the details of field evaporation. We use density functional theory to study field effects in ZnO clusters calculating the potential energy curves, the local field distribution, the polarizability, and the dielectric constant as a function of field strength. We confirm that, as in MgO, the HOMO-LUMO gap of a ZnO cluster closes at the evaporation field strength signaling field-induced metallization of the insulator. Following the structural changes in the cluster at the evaporation field strength, we can identify the field evaporated species, in particular, we show that the most abundant ion, Zn2+, is NOT post-ionized but leaves the surface as 2+ largely confirming the experimental observations. Our results also help to explain problems related to stoichiometry in the mass spectra measured in atom probe tomography.

Charge transport along proton wires

Using density functional theory we look at the quantum mechanics of charge transport along water wires both with free ends and donor/acceptor terminated. With the intermediate geometries in the DFT iterations we can follow the charge transfer mechanism and also construct the energy landscape explicitly. It shows activation barriers when a proton is transferred from one water molecule to the next. This, together with snapshots of intermediate geometries, leads to a justification and further elucidation of the Grotthuss mechanism and the Bjerrum effect. The charge transfer times and the conductivity of the proton wire are obtained in agreement with experimental results.Pacs82.39 Jn, 31.15 E

Field evaporation of insulators and semiconductors: Theoretical insights for ZnO.

We look at the new challenges associated with Atom Probe Tomography of insulators and semiconductors with regard to local fields inside and on the surface of such materials. The theoretical discovery that in high fields the band gap in these materials is drastically reduced to the point where at the evaporation field strength it vanishes will be crucial in our discussion. To understand Atom Probe results on the field evaporation of insulators and semiconductors we use density functional theory on ZnO clusters to follow the structural and electronic changes during field evaporation and to obtain potential energy curves, HOMO-LUMO gaps, field distributions, desorption pathways and fragments, dielectric constants, and polarizabilities. We also examine the effects of electric field reversal on the evaporation of ZnO and compare the results with Si.

Graphical methods for the analysis of shear-induced detachment of cells and microorganisms

Treating shear stress induced detachment of micro-organisms as a bond breaking mechanism, the authors present three intuitive graphical approaches to determine the relevant parameters in the Arrhenius rate equation, i.e., attachment energy, prefactor, and maximum shear stress. They demonstrate the methods with the detachment of polystyrene spheres and show that having three different methods presents the opportunity to check the consistency of the results.